Tue, December 01, 2020 Turnagain Pass Avalanche Forecast

Tue, December 1st, 2020

Alpine
Above 2,500'

High (4)

Treeline
1,000'-2,500'

High (4)

Below Treeline
Below 1,000'

High (4)

Alpine Above 2,500'		High (4)
Treeline 1,000'-2,500'		High (4)
Below Treeline Below 1,000'		High (4)

Danger Scale:

No Rating (0)

Low (1)

Moderate (2)

Considerable (3)

High (4)

Extreme (5)

Avalanche Problem 1

Storm Slabs
Almost Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Likelihood of Avalanches

Terms such as "unlikely", "likely", and "certain" are used to define the scale, with the chance of triggering or observing avalanches increasing as we move up the scale. For our purposes, "Unlikely" means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. "Certain" means that humans will be able to trigger avalanches on many slopes, and natural avalanches are expected.

Size of Avalanches

Avalanche size is defined by the largest potential avalanche, or expected range of sizes related to the problem in question. Assigned size is a qualitative estimate based on the destructive classification system and requires specialists to estimate the harm avalanches may cause to hypothetical objects located in the avalanche track (AAA 2016, CAA 2014). Under this schema, "Small" avalanches are not large enough to bury humans and are relatively harmless unless they carry people over cliffs or through trees or rocks. Moving up the scale, avalanches become "Large" enough to bury, injure, or kill people. "Very Large" avalanches may bury or destroy vehicles or houses, and "Historic" avalanches are massive events capable of altering the landscape.

Signal Word	Size (D scale)	Simple Descriptor
Small	1	Unlikely to bury a person
Large	2	Can bury a person
Very Large	3	Can destroy a house
Historic	4 & 5	Can destroy part or all of a village

More info at Avalanche.org

In the past 24 hours, the mountains have received 12-18” snow, equaling 1.1-1.7” snow water equivalent (SWE), and heavy precipitation is expected through the day. As of this morning, most of this precipitation has fallen as snow, but we are anticipating rain as high as 2500 feet throughout the day. Easterly winds were sustained 15-35 mph last night, with gusts as high as 110 mph. This combination of heavy snowfall and strong winds is rapidly loading start zones, and the snowpack will quickly loose strength when liquid water (read ‘rain’) is introduced. The snowpack does not react well to rapid change, and today we are making some big changes in a hurry. We can expect to see widespread natural activity during this storm, with large avalanches running far into runout zones. Avalanches failing within the new snow will be up to 4’ deep, and that doesn’t even take into account the potential for avalanches failing on weak layers deeper in the snowpack (see problem #2). At mid- to low elevations, these avalanches will involve wet snow, and they will have the potential to reach very long runout distances. There is no question about it, the current situation is dangerous, and it will get worse before it gets better. For now, we need to stay out of avalanche terrain.

Click Here for a video checking in from Seattle Ridge yesterday.

Avalanche Problem 2

Deep Persistent Slabs
Almost Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.

Likelihood of Avalanches

Size of Avalanches

Signal Word	Size (D scale)	Simple Descriptor
Small	1	Unlikely to bury a person
Large	2	Can bury a person
Very Large	3	Can destroy a house
Historic	4 & 5	Can destroy part or all of a village

More info at Avalanche.org

As if we didn’t already have enough to worry about with the storm snow issues, we also still have weak, faceted snow deep in the snowpack in many locations in our advisory area. This storm is placing a heavy burden on weak snow, and it is possible that we will see avalanches failing over 6′ deep in the snowpack. For today, we have plenty of clear signs indicating very dangerous conditions and avalanche terrain should be avoided entirely.

Tincan. We still don’t trust the weak snow near the ground. Photo: Eric Roberts. 11.30.2020

Weather

Tue, December 1st, 2020

Yesterday: In the past 24 hours, the mountains have received 12-18” snow, equaling 1.1-1.7” snow water equivalent (SWE), with lower totals at lower elevations. Easterly winds were sustained 15-35 mph last night, with gusts as high as 110 mph.

Today: As of this morning, most of this precipitation has fallen as snow, but we are anticipating rain as high as 2500 feet throughout the day. We will receive up to 12-18” more snow by the end of the day, equaling 1.0-1.2” water. Below 2000’, almost all of this will fall as rain, with .75″ rain expected in Girdwood. Winds will be 15-25 mph out of the southeast, with high temperatures in the low- to mid 30’s.

Tomorrow: This storm will taper off tonight with low temperatures hovering in the low- to mid 30’s. Lingering precipitation will be mixed rain and snow, with 1-2” snow and about 0.1” water. South winds will be 5-15 mph, and high temperatures will be in the low- to mid 30’s.

PRECIPITATION 24-hour data (6am – 6am)

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	28	17	1.7	74
Summit Lake (1400′)	18	6	0.6	27
Alyeska Mid (1700′)	30	13	1.15	73

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	19	ENE	34	110
Seattle Ridge (2400′)	24	ESE	15	43

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
05/22/23	Turnagain	Avalanche: Tincan	Nick D'Alessio
05/12/23	Turnagain	Avalanche: Tincan, Sunburst, Magnum, Cornbiscuit	Heather Thamm
05/07/23	Turnagain	Observation: Tincan – Bear Tracks	CNFAIC Staff
05/05/23	Turnagain	Observation: Seattle Ridge	AS/ WW Forecaster
05/02/23	Turnagain	Observation: Cornbiscuit	Schauer/ Sturgess Forecaster
05/02/23	Turnagain	Observation: Seward Hwy Turnagain Pass	Joel Curtis
04/30/23	Turnagain	Observation: Magnum	Ayla, Kit Crosby, Barton
04/29/23	Turnagain	Observation: Tincan	John Sykes
04/28/23	Turnagain	Avalanche: Taylor Pass/Pastoral	Schauer/ Creighton Forecaster
04/28/23	Turnagain	Avalanche: Tincan	Andy Moderow

Riding Areas

Updated Thu, June 01st, 2023

Status of riding areas across the Chugach NF is managed by the Glacier and Seward Ranger Districts, not avalanche center staff. Riding area information is posted as a public service to our users and updated based on snow depth and snow density to prevent resource damage at trailhead locations. Riding area questions contact: mailroom_r10_chugach@fs.fed.us

Area	Status	Weather & Riding Conditions
Glacier District
Johnson Pass	Closed	Closed May 15.
Placer River	Closed	Closed May 15.
Skookum Drainage	Closed	Will close to motorized use on April 1, 2023 per the National Forest Plan.
Turnagain Pass	Closed	Will close on June 1.
Twentymile	Closed	Scheduled closure May 1.
Seward District
Carter Lake	Closed	Scheduled closure May 1.
Lost Lake Trail	Closed	Scheduled closure May 1.
Primrose Trail	Closed	Scheduled closure May 1.
Resurrection Pass Trail	Closed	Closed to motorized use for the 2022/23 winter season per Forest Plan. Open next season.
Snug Harbor	Closed	Extended opening until May 15.
South Fork Snow River Corridor	Closed	Scheduled closure May 1.
Summit Lake	Closed	Scheduled closure May 1.